The present invention relates to the production of calcium by the thermal reduction of calcium oxide in the presence of a molten oxide slag. More particularly, this invention relates to the production of calcium by contacting or reacting an aluminum reducing agent with a molten calcium-aluminum oxide slag or with calcium oxide in the presence of such slag.
Several processes for the production of calcium by thermal reduction are known. These processes generally operate to react calcium oxide in the solid state with a metallic reducing agent, such as silicon or aluminum or mixtures or alloys thereof.
U.S. Pat. No. 2,464,767 to Pidgeon et al (hereinafter referred to as the Pidgeon Process) describes a solid state reaction process for the production of calcium. In carrying out this process, finely ground calcium oxide ore, preferably high calcium limestone, is mixed with finely ground aluminum and formed into small dense briquettes having a density of about 2.2. In a preferred embodiment, an excess of the theoretical amount of aluminum required to reduce the calcium oxide (i.e., an excess of from 5 to 20%) is mixed in the briquettes. The briquettes are charged to a gas-fired or electrically heated retort having a reaction zone and a water-cooled condensation zone. The retort is evacuated and heated so that the temperature in the reaction zone is about 1170.degree. C. Typically, the pressure in the reaction zone is less than 10 microns (i.e., 0.01 torr). Under these conditions, the aluminum reacts with the calcium oxide ore to produce calcium vapor which is conducted to the condensation zone, where it is condensed as a solid.
Another thermal reduction process utilizing a solid state reaction is described in U.S. Pat. No. 2,448,000 of Kemmer. This process is described in the context of producing magnesium, but it is mentioned that the process is applicable to calcium and other alkaline earth metals as well. The process utilizes aluminum as the reducing agent, but also requires the addition of a moderating agent to the reaction zone. This moderating agent consists of aluminum nitride, a mixture of aluminum nitride, aluminum carbide and aluminum oxide or a mixture of ferrosilicon, aluminum nitride, aluminum carbide and aluminum oxide. In one embodiment of this process, there is used as a combined reducing agent and moderating agent "the dross which is obtained in melting and subsequently casting aluminum or aluminum alloys", provided that the dross contains about 0.5 to 10% by weight aluminum nitride.
U.S. Pat. No. 4,240,825 to Tamas describes another solid state, thermal reduction process for the production of calcium. According to the process, calcined lime is reduced under a pressure lower than 10 torr and at a temperature of 1300.degree. to 1600.degree. C. with a reducing agent containing silicon and aluminum in a weight ratio of 4:1 to 1:1, wherein the total amount of silicon and aluminum in the reducing agent may vary between 50 and 100% by weight. 100 to 200 parts by weight of the reducing agent are applied to convert 700 to 1000 parts by weight of calcined lime. The process provides cement as by-product instead of a useless slag.
Calcium is presently produced commercially by solid state processes similar to the previously described Pidgeon Process. The retorts in which the calcium is produced are relatively small and, thus, yields per retort are rather low. As a result, a significant number of retorts are needed for commercial production. Moreover, the operation and maintenance of such a number of retorts require a significant labor force.
Accordingly, it would be most desirable if a higher yielding, less labor intensive process for producing calcium were available.